You will find out pretty quickly if it is way off just by trying it and seeing what you get. Otherwise you need to test your film to find out your proper speed anyway. I had one I had been using heavily (for everything) for 30 years, it was scarred up and a little rattly and if I pushed the trigger in too far it stopped reading. I found someone selling one in mint condition on craigs list for (probably stolen) cheap price. I checked them against one another and found they were a third off... in linear fashion. I took them to a local camera repairman who was able to check them against his source and found the old one was correct so he adjusted the new one to match it with out a lot of trouble and charged me 40.00